Re: [PATCH v2 net-next 2/2] kcm: Check if sk_user_data already set in kcm_attach

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2018-01-24 at 12:35 -0800, Tom Herbert wrote:
> This is needed to prevent sk_user_data being overwritten.
> The check is done under the callback lock. This should prevent
> a socket from being attached twice to a KCM mux. It also prevents
> a socket from being attached for other use cases of sk_user_data
> as long as the other cases set sk_user_data under the lock.
> Followup work is needed to unify all the use cases of sk_user_data
> to use the same locking.
> 
> Reported-by: syzbot+114b15f2be420a8886c3@syzkaller.appspotmail.com
> Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
> Signed-off-by: Tom Herbert <tom@quantonium.net>
> ---
>  net/kcm/kcmsock.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
> index 7632797fb68e..4a8d407f8902 100644
> --- a/net/kcm/kcmsock.c
> +++ b/net/kcm/kcmsock.c
> @@ -1410,9 +1410,18 @@ static int kcm_attach(struct socket *sock, struct socket *csock,
>  		return err;
>  	}
>  
> -	sock_hold(csk);
> -
>  	write_lock_bh(&csk->sk_callback_lock);
> +
> +	/* Check if sk_user_data is aready by KCM or someone else.
> +	 * Must be done under lock to prevent race conditions.
> +	 */
> +	if (csk->sk_user_data) {
> +		write_unlock_bh(&csk->sk_callback_lock);
> +		strp_done(&psock->strp);

Although it seems psock->strp->stopped wont be set ?

We should hit WARN_ON(!strp->stopped);